A power supply circuit using a typical charge pump circuit as typified by a Dickson charge pump has advantages over a switching regulator requiring a coil in that it can have a small circuit size and archive low noise. Although a typical charge pump circuit has a function to step up an input voltage, it does not have a function to step down the input voltage. For this reason, it may not suitable for use (e.g., for a vehicle) where a constant voltage needs to be generated from an input voltage which fluctuates over a wide range. Some techniques were proposed to overcome this disadvantage by modifying a typical charge pump circuit.
In JP H10-337003A corresponding to U.S. Pat. No. 5,841,725, when an output voltage is higher than a target voltage, a charge pump is stopped so that the output voltage can decrease, and when the output voltage is smaller than the target voltage, the charge pump is started so that the output voltage can increase. In JP 2006-014581A corresponding to US 2005/025889A, when an output voltage is higher than a target voltage, a dropper power supply (i.e., series regulator) is driven to perform a step-down operation, and when the output voltage is smaller than the target voltage, a charge pump circuit is driven to perform a step-up operation.
In such structures, when switching occurs in a sequence of operations (i.e., switching between ON and OFF of the charge pump, switching between the dropper power supply and the charge pump), its output characteristics may have a step, so that linearity of an output voltage may be degraded accordingly. The degradation in linearity of the output voltage becomes significant if a destination circuit to which the output voltage is inputted has characteristics varying depending on its input voltage, i.e., if the destination circuit requires high accuracy for its input voltage. Therefore, the conventional structures may not be suitable for a power supply circuit.
In JP 2010-045943A, a duty cycle of a clock signal for turning ON and OFF a switch of a charge pump is changed so that a detection value of an output voltage can be equal to a target value. However, even in this structure, it is difficult to ensure a wide range over which the output voltage can change. Therefore, if an input voltage fluctuates over a wide range, it may be impossible to generate a predetermined constant output voltage from the input voltage.
In JP 2003-348822A corresponding to US 2004/0017247A, the amount of an increase in a voltage is reduced by switching a power voltage for a clock buffer of a charge pump in small steps using a D/A converter, so that noise can be reduced. However, in this structure, it is impossible to perform a step-down operation, and by extension, to generate a predetermined constant output voltage. Further, since its circuit size becomes large, its manufacturing cost may become large.